The present invention relates to coated microporous membranes which exhibit improved wettability and can be used in alkaline storage batteries and particularly in batteries having electrode systems containing zinc and silver electrodes, e.g., nickel-zinc and silver-zinc, and a process for making the same.
Recent developments in the area of open celled microporous polymeric films, exemplified by U.S. Pat. Nos. 3,839,516; 3,801,404; 3,679,538; 3,558,764; and 3,426,754, have instigated studies to discover applications which could exploit the unique properties of these new films. Such films which are in effect a gas-breathing water barrier can be used as vents, gas-liquid transfer mediums, battery separators and a variety of other uses.
One disadvantage of these films, which in the past has limited the number of applications to which they may be put, has been their hydrophobic nature. This is especially true when polyolefinic films, a preferred type of polymeric material often employed in the manufacture of microporous films, are employed. Because these films are not "wetted" with water and aqueous solutions they could not be used advantageously in such logical applications as filter media electrochemical separator components and the like.
Several proposals have been put forth in the past to overcome these problems such as exemplified by U.S. Pat. No. 3,853,601; and Canadian Pat. No. 981,991, which utilize a variety of hydrophilic surfactant impregnants. Such surfactant impregnants while imparting hydrophilicity to the microporous film do not maximize the properties of said films when employed as battery separators.
More specifically, a battery separator is a critical component of a battery. A battery is comprised of one or more electrolytic cells enclosed by a housing. Each cell includes two electrical terminals or electrodes, the anode and the cathode. The electrodes are immersed in a conducting medium, the electrolyte. Electrical current flows between the electrodes. This electrical current results from the flow of electrons across a circuit external to the electrolyte. Just as electrons, flow across the external circuit so do ions, i.e., charged species, flow in the electrolyte. Although it is absolutely essential to the production of an electrical current that ions flow between electrodes in the electrolyte, it is usually detrimental in a battery for ionic species derived from a respective electrode to flow to the electrode of opposite charge with respect to said ionic species. This interferes with the efficiency of the battery. To prevent deleterious ionic flow of one or more ionic species between terminals is a function of a battery separator. More specifically, a battery separator is disposed in the electrolytic cell between the anode and cathode of the electrolyte to prevent or retard deleterious ion migration.
The above description suggests the type of material that should ideally be used as a battery separator. An excellent battery separator is one which has pore openings which are small enough to prevent large ionic species, such as large electrode derived ions to flow through its pores yet large enough to permit the flow of electrolyte derived ions such as K.sup.+ and OH.sup.- through these pores to reach the electrode of opposite charge in relation thereto. Similarly, the battery separator should be of minimum thickness in view of the well known fact that the flow of ions across a battery separator is inversely proportional to the thickness of the separator.
Another requirement of a battery separator suggests itself when one considers that the electrolytes employed in most battery applications are highly basic or acidic. A good battery separator should be inert, to these highly corrosive materials.
Still another requirement for a good battery separator is that the separator be rapidly wetted by the electrolyte employed. In view of the fact that essentially all of the electrolytes currently utilized are aqueous solutions, this requirement necessitates that the battery separator be hydrophilic. The battery separator must be totally and rapidly wetted so as to provide a continuous ionic path on either side of the battery separator to permit the flow of certain ions therethrough. An analogy can be drawn to an electrically conducting wire. A break in the wire cuts off the flow of electrons. So, in the case of an electrolyte, the non-wetting of a portion of a battery separator effectively cuts off the path for ionic flow over the non-wetted area, thus, cutting down on the output of the battery.
Seemingly, the aforedescribed requirements, if satisfied, should be enough to produce a satisfactory battery separator. Unfortunately, in addition to the above-described criteria for battery separators, additional properties should also be possessed by the same depending on the type of electrode system employed therein. For example, while a nickel-zinc battery has one of the best initial energy-to-weight and power-to-weight characteristics of known batteries, the same exhibits poor cycle life, i.e., the number of charge and discharge cycles which a battery can undergo before it no longer is capable of performing its intended function.
The poor cycle life of nickel-zinc batteries is especially troublesome for a variety of reasons. This problem is associated with any secondary battery which employs zinc as the anode and an alkaline electrolyte, because of the high solubility of the oxidation products thereof, namely, ZnO or Zn(OH).sub.2.
The short cycle life of batteries employing zinc anodes is attributed to premature cell failures which can be characterized as being catastrophic or gradual. Catastrophic cell failures are believed to be due to internal shorting of the cell by the growth of zinc dendrites which form a bridge between the electrodes.
For example, the nickel-zinc battery is based on the following half-cell reactions: EQU 2NiO(OH).revreaction.2Ni(OH).sub.2 +2H.sub.2 O+2OH.sup.- -2e.sup.- EQU Zn.revreaction.ZnO+H.sub.2 O+20H.sup.- +2e.sup.-
The reversible reactions are written so that the discharge cycle reads from left to right. The zinc half-cell reaction as written above, however, is an oversimplification since the oxidized form of zinc exists as a mixture of ZnO, Zn(OH).sub.2 and Zn(OH).sup.--.sub.4. The zincate ion (Zn(OH).sup.--.sub.4) is soluble and contributes to the complexity of cell performance.
When a battery employing a zinc anode is charged, the above-described reaction reverses and zinc is formed. Ideally, the zinc which is formed is redeposited on the zinc anode. However, some of the zinc which is produced in a charging sequence characterized by a high current density gives rise to formation of zinc dendrites which tend to bridge out from the zinc anode and connect up with the cathode. Even when a battery separator is inserted between the electrodes the zinc dendrites can actually penetrate the separator over a number of charging cycles leading to catastrophic cell failure.
The gradual, but unacceptable rapid loss of cell energy capacity occurs more frequently with repeated deep discharge cycling wherein the active mass of zinc anode is almost completely depleted. This gradual loss of energy capacity is related to pore plugging, other deterioration in the separator, and to shape change in the zinc electrode. Pore plugging is caused by the precipitation of various soluble zincate ions (e.g., Zn(OH).sup.--.sub.4) which are formed during discharge but which become insoluble when the load is removed from the battery. If the soluble zincate ions are within the pores when precipitation occurs, the pores become plugged thereby reducing the efficiency of the separator.
The shape change of the electrode results from the fact that the zinc is not redeposited during charging at the location where it has been oxidized during discharging but is redeposited and concentrated instead in that part of the cell where the current density is greatest. This uneven redepositing of the zinc ions causes densification of the electrode and reduces its effective surface area. The uneven buildup of the zinc causes the electrode to swell in thickness.
The harmful effects of the shape change in the electrode are further aggravated when the separator also changes shape by swelling in three dimensions. The combined shape change of the electrode and the separator creates pressures in the latter which can rupture the same resulting in cell failure.
Many attempts have been made to prevent the formation of dentritic zinc and electrode shape change or to avoid the damaging consequences thereof. Thus, some success has been achieved with a pulsating charging current, electrolyte additives, electrolyte circulation, and the use of special separators.
More specifically, a great deal of attention is being given to the design of battery separators.
As may be gleaned from the above discussion, separator performance is one of the keys to the durability of secondary batteries, particularly zinc electrode containing batteries. The separator's ability to control the flow of electrolyte components plays a limiting role in determining maximum power to weight ratio, in maintaining a uniform zinc electrode shape, and in retarding the diffusion of certain ions, e.g., zincate to the cathode. Furthermore, initial electrolyte flow properties should not be altered by the accumulation of ZnO within the pores of the separator. Moreover, the separator is expected to survive the harsh oxidative alkaline environment of the electrolyte in the vicinity of the cathode for the target life of the cell.
Battery separators employed in the past can be segregated into two basic categories, namely, those which are diffusion limited and those which are limited by a mass transport mechanism.
A diffusion limited membrane as defined herein is one in which the exchange of soluble ionic species between one side of the membrane and the other, which occurs when the membrane is employed as a battery separator, does so as a result of the affinity of said ionic species for the membrane and the rate of said exchange is limited by the concentration gradient of said soluble ionic species which exists in the solution present at each side of the membrane for a given membrane thickness. The affinity of the ionic species for the membrane is similar to the affinity which a solute has for a solvent.
For example, cellulosic films (e.g., cellophane, sausage casing) have been the most common diffusion limited membranes employed as separators for a nickel-zinc cell. When such films are immersed in an electrolyte such as an aqueous solution of KOH they will absorb electrolyte and water. It is believed that the exchange of further electrolyte ions through the film occurs by a continuous dynamic process of intermittent attraction between the film and the electrolyte ions. During the course of this intermittent attraction the electrolyte ions exchange one site in the film for another and gradually make their way through the interior of the film until they reach the other side thereof. The pore size of a diffusion limited membrane is within the order of the molecular dimensions of the electrolyte and is therefore too small to permit a free or viscous flow of ions therethrough when an external force or pressure is applied to the electrolyte solution to force it through the film. It is only by increasing the concentration gradient that the rate of exchange can be increased. Thus, while the ionic exchange between the planar surfaces of the membrane is also limited by the thickness of the membrane, and the solubility of the ionic species, when these parameters are fixed it is the limitation on the rate of exchange of ionic species imposed by the concentration gradient which particularly characterizes diffusion limited membranes for purposes of the present invention. The dependence of the ionic exchange rate of diffusion limited membranes on the concentration gradient is disadvantageous since there is a limit to the concentration gradient which can be employed under high current density and in a real cell environment.
This necessarily limits the rate of ionic exchange which in turn limits charging and discharging rates of batteries which employ diffusion limited membranes as separators. While the disadvantages of diffusion limited membranes with respect to the functional relationship between rate of ionic exchange and concentration gradient can be compensated for to some extent by reducing the thickness of the membrane, this remedy is impractical since such membranes must possess a substantial thickness to exhibit sufficient mechanical strength and structural integrity necessary for handling and cell manufacture. Thus, the use of diffusion limited membranes as a battery separator imposes inherent limitations on charging and discharging rates on batteries employing the same. If the charging or discharging rate is too fast, local depletions of hydroxide ion occur, leading to electro-osmotic pumping and convective flow of electrolyte, which cause erosion and lateral shape changes on the zinc electrodes. Furthermore, cellulosic films are dimensionally unstable leading to rupture for the reasons noted above.
A mass transport limited membrane achieves exchange of ionic species not only by a diffusion mechanism (due to the presence of a concentration gradient) but also by actual transport of the ionic species through porous channels which are large enough to permit unimpeded viscous flow of the ions therethrough. Consequently, a mass transport limited membrane is limited the pore volume and thickness of the film. The exchange of ionic species between the planar surfaces of a mass transport limited membrane is much faster than would otherwise occur in a diffusion limited membrane. Consequently, the inherent limitations on charging and discharging rates imposed by the use of a diffusion limited membrane as a battery separator are absent. A mass transport limited membrane is therefore characterized by the ability to increase the flow of a liquid, such as an electrolyte, therethrough in response to an increase in pressure applied to one side of the membrane. A diffusion limited membrane will not exhibit this response without rupturing. For example, a microporous film prepared in accordance with Example 1, herein, will exhibit a flow of ethanol therethrough of 0.05 cc/cm.sup.2 /min. at a pressure drop of 760 mmHg while cellophane exhibits substantially no measurable flow of the same at the same pressure.
Aside from the aforedescribed limitations associated with cellulosic separators as a result of their being diffusion limited, the most limiting shortcoming of these separators is their degradation in the cell environment. Oxidation of the cellulose within the cell results in the formation of CO.sub.2 as one of the products of oxidation which reacts with the electrolyte cation, such as potassium, forming for example potassium carbonate. The potassium carbonate increases the internal resistance of the cell. Since the CO.sub.2 formation is a manifestation of the degradation of the membrane, the membrane can rupture permitting transfer of oxygen gas formed at the positive electrode upon overcharging thereby lowering of the cell capacity, inducing loss of negative electrode capacity, and increasing the risk of thermal runaway. Eventually the physical failure of the degraded cellulosic separator terminates the cell's life.
Various approaches used to cope with the degradation problem all involve compromises of cell characteristics and/or cost. For example, electrolyte concentrations above 40% KOH are used with cellulosic separators to reduce the degradation rate. However, at 31% KOH, where the cell's internal resistance would be the lowest, the degradation rate of cellophane is unacceptable.
Multiple layers of cellulose separators permit additional cycles, but at increased separator cost and weight gain, and an increase in internal resistance. Furthermore, due to inherent swelling characteristics of cellulose separator films it is difficult to pack several of such films in a space efficient fashion.
For example, U.S. Pat. No. 3,894,889 (see also U.S. Pat. No. 3,980,497) is directed to a process for preparing a laminated separator. In one embodiment the laminated separator comprises two bibulous, non-membranous separator layers, e.g., high grade microporous cellulosic filter papers, laminated together with a layer of gelling agent, such as cellulose acetate. In a second embodiment a semi-permeable membrane such as polyethylene is sandwiched between two gel coated bibulous layers. The sandwiched laminate is then hot pressed to form an integral smooth separator. The resulting laminate structures allegedly provide strength and protection to the semi-permeable membrane layer during battery cell assembly and operation. Thus, the gelling agent is used merely as a glue and cellulose acetate is not suggested as a means for rendering hydrophobic microporous open-celled membranes hydrophilic or for reducing pore plugging which occurs for example in a nickel-zinc cell. The laminate also results in an undesired weight gain in the separator and because of the thickness of the laminate separator it cannot be packed in a minimum of space. Moreover, the aforenoted laminated separator does not overcome the basic problem of degradation of the microporous cellulosic filter papers.
Microporous polypropylene which has a pore size in the order of 200 .ANG. is an example of a mass transport limited separator wherein the electrolyte balance is maintained by mass transport thereof through the pores. Because of the ease of electrolyte transport, concentration gradients do not build up during high rate charge and discharge, and convective flows and electroosmotic pumping effects are reduced. Furthermore, polypropylene is chemically inert in the cell environment, thus permitting operation at KOH concentrations favoring minimum cell internal resistance. Such mass transport limited films are not without their own disadvantages, however. For example, the pore structure of certain microporous films permits the transfer of zincate to the nickel compartment. As described above, after repeated cycling, zinc and zinc oxide accumulate in the separator. Furthermore, such microporous films can be penetrated by zinc dendrites which leads to catastrophic failure of the cell.
Attempts to circumvent the dendrite shorting problem using metal barrier layers are illustrated in U.S. Pat. Nos. 3,539,374; 3,539,396; 3,970,472; 4,039,729.
None of the above patents employs a polymer (e.g., cellulose acetate) coated microporous membrane to reduce dendrite shorting.
U.S. Pat. No. 4,172,183 describes an alkaline primary cell which employs two separator layers one being microporous propylene known as Celgard.TM. and disposed on top of this layer is one or more non-fibrous cellulose membranes such as cellophane. While the thickness of the cellophane layer is not disclosed, the fact that it is initially employed as an integral film suggests that its thickness is relatively substantial for handling purposes. Consequently, the double layer separator is distinct from the coated microporous films of the subject invention wherein the coating is substantially thinner than can be achieved using a separate cellophane film. Furthermore, there is no mention of the ability of cellophane to improve the wetting characteristics of the Celgard.TM. microporous film.
The search has therefore continued for a means of rendering normally hydrophobic microporous films highly hydrophilic and at the same time improving their capacity to act as a battery separator in primary and secondary batteries. The present invention was developed in response to this search.
It is therefore an object of the present invention to provide a hydrophilic microporous membrane which is rapidly wettable in aqueous, preferably alkaline aqueous solutions.
It is another object of the present invention to provide a process for rendering a normally hydrophobic microporous membrane hydrophilic.
It is still another object of the present invention to provide a hydrophilic microporous membrane which is substantially dimensionally stable in an alkaline solution.
It is a further object of the present invention to provide a hydrophilic microporous membrane which reduces the plugging of its pores by electrode derived ions, such as by zinc oxide, when employed as a battery separator for a nickel-zinc battery.
It is still another object of the present invention to provide a hydrophilic microporous membrane which retards the migration of silver ions therethrough when employed as a battery separator in a silver-zinc battery.
It is another object of the present invention to provide a secondary electrochemical cell containing a hydrophilic microporous film battery separator.
These and other objects and features of the invention will become apparent from the claims and from the following description of the preferred embodiments of the present invention.